The present invention generally relates to the measuring and monitoring of blood pressure. More specifically, embodiments may apply the theory of applanation tonometry for the measurement of blood pressure. Some embodiments provide a method for measuring mean arterial pressure. Some embodiments provide a device that may be worn by a user that may non-invasively measure and monitor absolute arterial pressure of a user.
Elevated blood pressure (a.k.a. hypertension) is a major risk factor for cardiovascular disease. As a result, blood pressure measurement is a routine task in many medical examinations. Timely detection of hypertension can help inhibit related cardiovascular damage via accomplishment of effective efforts in treating and/or controlling the subject's hypertension.
A person's blood pressure is a continuously changing vital parameter. As a result, sporadic office blood pressure measurements may be insufficient to detect some forms of hypertension. For example, hypertension can occur in a pattern that evades detection via isolated office blood pressure measurement. Common hypertension patterns include white coat hypertension (elevated only during a limited morning period of time), borderline hypertension (fluctuating above and below definitional levels over time), nocturnal hypertension (elevated only during sleeping hours), isolated systolic hypertension (elevated systolic pressure with non-elevated diastolic pressure), and isolated diastolic hypertension (elevated diastolic pressure with non-elevated systolic pressure). To detect such hypertension patterns, it may be necessary to perform additional blood pressure measurements over time to obtain a more complete view of a person's blood pressure characteristics. Although continuous measurement of blood pressure can be achieved by invasive means, for example, via an intra-arterial pressure sensing catheter, noninvasive blood pressure measurement approaches are more typically used.
Current noninvasive blood pressure measurement approaches include ambulatory and home blood pressure measurement strategies. These strategies provide such a more complete view of a person's blood pressure characteristics and are often employed in recommended situations. Ambulatory blood pressure measurement is performed while the person performs daily life activities. Currently, ambulatory blood pressure measurements are typically performed every 20 to 30 minutes using brachial oscillometric blood pressure measurement cuffs. Ambulatory blood pressure measurement may be recommended where there is large variability in office blood pressure measurements, where a high office blood pressure measurement is made in a person with otherwise low cardiovascular risk, when office and home blood pressure measurements vary, where resistance to drug treatment of blood pressure is noted or suspected, where hypotensive episodes are suspected, or where pre-clampsia is suspected in pregnant women. Home blood pressure measurement includes isolated self-measurements performed by a person at home. Home blood pressure measurements may be recommended where information is desired regarding the effectiveness of blood pressure lowering medication over one or more dose-to-dose intervals and/or where doubt exists as to the reliability of ambulatory blood pressure measurement.
Current ambulatory and home blood pressure measurement approaches, however, fail to provide continuous measurement of blood pressure. Additionally, when an oscillometric blood pressure measurement cuff is used to monitor a person's blood pressure when sleeping, the intermittent inflation and deflation of the cuff can disturb the person's sleeping pattern, thereby harming the subject to some extent and potentially changing the person's sleeping blood pressure. Thus, convenient and effective approaches for noninvasive continuous measurement of blood pressure remain of interest.
According to the theory of arterial tonometry, the pressure in a superficial artery with sufficient bony support, such as the radial artery, may be accurately recorded during an applanation sweep when the transmural pressure equals zero. An applanation sweep refers to a time period during which pressure over the artery is varied from overcompression to undercompression or vice versa. At the onset of a decreasing applanation sweep, the artery is overcompressed into an occluded state, so that pressure pulses are not recorded. At the end of the sweep, the artery is undercompressed, so that minimum amplitude pressure pulses are recorded. Within the sweep, it is assumed that an applanation occurs where the arterial wall is flattened and transmural pressure turns to zero, and the arterial pressure is perpendicular to the surface and is the only pressure detected by a tonometer sensor.
FIG. 1 illustrates a method of measuring blood pressure using applanation tonometry. Here, a pressure transducer 1 is urged against the skin 2 of a user with an applanation force 3. The applanation force 3 and pressure transducer 1 applanate the target artery 4 such that the arterial wall tension 5 is parallel to the pressure transducer surface 6 and the arterial pressure 7 is perpendicular to the surface 6. Where the target artery 4 is applanated in such a manner, the arterial pressure may be measured by transducer 1. The target artery 4 may be supported by bone 8 and adjacent muscle 9. The target artery 4 may be the radial artery of the user and the bone 8 may be the radial bone.
FIG. 2 illustrates an exemplary cross-section of a wrist. As mentioned above, the radial artery is generally targeted in arterial applanation tonometry given its position adjacent the radial bone (radius). However, finding an ideal or preferred location for applanation of the radial artery can be difficult given its relative size. Compounding this problem is the fact that human anatomy varies from person to person and may change based on a person's height, weight, gender, etc. Accordingly, targeting the radial artery and identifying a preferred applanation location and orientation can be a challenge.
Some prior devices and methods have used a single pressure sensor for applanation of the target artery. Such methods and devices, however, require first locating a desired applanation location and then positioning of a pressure sensor at the desired location. As discussed above, this may not be a simple task given the size of the target artery and the variation in body anatomies. Further, some prior designs and methods have also required the use of wrist harnesses which orient the user's wrist in a preferred orientation prior to applying a pressure sensor to the target artery. These harnesses are bulky and inconvenient. Additionally many of the prior devices and methods require the assistance of a trained health care technician or are otherwise carried out in a clinic setting. Such devices and methods are inapplicable for day-to-day use by the general public.
In addition, given the complex anatomy of the wrist, as illustrated in FIG. 2, issues with signal processing and calibration have been challenging. While prior devices and methods have obtained pressure signals from patients, it has been challenging to convert these pressure signals into meaningful data. This may be further complicated if, as discussed above, the pressure sensor is applied to a less than ideal position where the sensor is not over the target artery or applied at an orientation where the pressure signal is not perpendicular to the surface of the sensor.
Accordingly, while applanation tonometry devices and methods have been provided, improvements in continuous and/or non-invasive blood pressure monitoring may be still be desired. For example, methods and devices for easily identifying a preferred applanation region and ensuring at least one pressure sensor is preferably placed adjacent the target artery may be of interest. These methods and devices may reduce issues with signal processing as a preferred applanation location may be identified and at least one pressure sensor is preferably placed such that a received pressure signal may be stronger and may require less processing. Further, identification of the preferred location may be carried out autonomously or may be identified by interpreting pressure signals from a plurality of locations. Pressures signals may be received from each location or region about the target artery selectively, simultaneously, sequentially, in subsets or the like. Further methods and devices that provide a convenient manner and/or less bulky device for measuring or monitoring blood pressure may increase the adoption of such techniques and may facilitate an increase in non-clinic setting measurements and monitoring of blood pressure.